Duplex stainless steel

ABSTRACT

A duplex stainless steel having a good combination of hot workability, corrosion resistance and cold formability, said duplex stainless steel consisting essentially of, in weight percent, carbon 0.12 max., nitrogen 0.12 max., manganese 8 to 16, silicon 1.0 max., nickel 1.0 max., chromium 15 to 20, molybdenum 0.6 to 2.0, carbon + nitrogen 0.06 to 0.14 and the balance iron, the chromium equivalent of 13.5 to 16.0 in accordance with the formula:

O United States Patent 1191 [111 3,861,908

Bressanelli Jan. 21, 1975 DUPLEX STAINLESS STEEL 3,152,934 10/1964 Lula 75/128 A 3,736,131 5/1973 Es 75/128 A X [75] Inventor Jerome Bressanell" 3,756,807 9/1973 Hoziiino 75/128 A Allegheny County, Pa. [73] Assignee: Crucible Inc., Pittsburgh, Pa. Primary Examiner-C Love" [22] ed ug 20 9 Assistant ExaminerArthur J. Steiner 21 Appl. No.: 389,832 ABSTRACT A duplex stainless steel having a good combination of [52] U S Cl 75/126 B 75/126 C 75/126 J hot workability, corrosion resistance and cold form- "75/128 A 75/128 N 75/128 ability, said duplex stainless steel consisting essentially [51] Int Cl 39/14 of, in weight percent, carbon 0.12 max., nitrogen 0.12 [58] Fie'ld C 128 A max., manganese 8 to 16, silicon 1.0 max., nickel 1.0 75/126 128 128 max., chromium 15 to 20, molybdenum 0.6 to 2.0, carbon nitrogen 0.06 to 0.14 and the balance iron, [56] References Cited the chromium equivalent of 13.5 to 16.0 in accor- UNITED STATES PATENTS dance with the formula:

Chromium equivalent in percent %Cr %Si -1- %Mo 2,225,440 12/1940 Becket 75/128 A 2,357,885 9/1944 Franks 75/126 B 30 (%C+%N)' 3,112,195 11/1963 Souresny 75/126 B 3 Claims, 7 Drawing Figures mmm 1 ms SHEEI H 0F 6 Q Q was @395 x RG33 gmmma 9 9 9 9 9 r! A I REM vi SE a 0 (395 50861571 5170/1) 7l7/JA/5'J0d PATENTED JANZI I975.

SHEET 5 BF 6 PATENTEI] JANZI i975 SHEET 8 OF 6 DUPLEX STAINLESS STEEL For applications involving the manufacture of trim for motor vehicles, such as hubcaps, wheelcovers, bumpers, and the like, it is necessary to have a steel that possesses good hot-workability and yet may be cold formed to the desired trim configurations. In addition, during use in the corrosive environments incident to motor vehicle operation it is necessary that the steel provide good corrosion resistance. Further, in view of 10 the increasing cost and scarcity of the element nickel, it is desirable to achieve these properties in an alloy that is essentially nickel free of where nickel is not present in more than what is typically termed a residual amount.

It is accordingly an object of the present invention to provide a substantially nickel-free, duplex stainless steel containing substantially equal amounts of ferrite and austenite after annealing at temperatures typically within the range of l,850 to 2,050F and characterized by good hot workability, good cold formability and corrosion resistance.

These and other objects of the invention, as well as a more complete understanding thereof may be obtained from the following description, specific examples and drawings, in which:

FIG. I is a graph showing the effect of carbon and nitrogen on the room-temperature tensile ductility;

FIG. 2 is a photograph showing the effect of various contents of carbon and nitrogen on the hot-workability of various alloy samples;

FIG. 3 is a graph showing the effect of manganese on the longitudinal tensile properties;

FIG. 4 represents anodic polarization curves for various steel compositions in a one normal sulfuric acid solution containing 0.5 moles of sodium chloride per liter;

FIG. 5 is a photograph showing the pitting characteristics of various stainless steel samples with varying contents of molybdenum;

FIG. 6 comprises photographs of four stainless steel samples each with varying molybdenum contents witth respect to corrosion resistance; and

FIG. 7 is a graph showing the relationship of austenite and ferrite content on the hot workability and cold workability, with chromium equivalent %Cr %Si %Mo 5Ni 30 (%C+%N).

As will be demonstrated and discussed in detail hereinafter the hot workability of the alloy is dependent upon restricting the combined carbon plus nitrogen contents. In addition, for this purpose, the composition with respect to the austenite and ferrite promoting elements must be balanced to achieve a chromium equivalent wherein the alloy has substantially equal amounts of ferrite and austenite. Hot workability is promoted by the presence of ferrite. Conversely hot workability is impaired by the presence of austenite. Austenite, however, promotes cold formability and thus must be present in the alloy for this purpose in combination with the ferrite necessary for hot workability to result in what is commonly termed a duplex alloy wherein the structure contains both austenite and ferrite. For purposes of corrosion resistance a minimum chromium content of 15 percent by weight is required. For this purpose, in combination with chromium, the alloy must contain molybdenum in an amount of at least 0.6 percent by weight; however, molybdenum must be restricted with regard to the maximum amount present as excessive amounts will have a detrimental effect on hot workability.

In accordance with the above the following constitutes a broad and preferred range of composition, in percent by weight, for a duplex stainless steel in accordance with the invention:

Chromium Equivalent %Cr %Si %Mo %Ni 30 (%C+%N) For applications of elevated temperature use requiring a high level of oxidation resistance the aboverecited composition limits may be modified to provide a silicon content of up to about 3 percent by weight.

By way of specific examples and to establish the composition limits for the duplex stainless steel of the invention compositions presented in Table II were produced.

TABLE I1 COMPOSITIONS OF EXPERIMENTAL ALLOYS Heal Grade C Mn Si Ni Cr Mo N l Duplex .092 5.87 .30 0.29 17.03 .038

8 do. .084 11.50 .57 0.31 16.79 .25 .037 1E57 do. .045 11.89 .30 0.33 17.23 .25 .034 1E59 do. .069 12.06 .33 0.33 17.86 .25 .037 1561 do. .095 12.13 .34 0.33 17.45 .25 .041 [E62 do. .095 12.23 .31 0.36 17.97 .25 .053 716 do. .10 11.80 .49 0.20 17.20 .16 .059 2314 do. .084 12.62 .18 0.47 18.07 .25 .040 2309 do. .084 12.34 .25 0.41 17.63 .46 .035 2310 do. .082 12.56 .24 0.41 17.21 .71 .036 2311 do. .081 12.59 .26 0.39 16.81 .90 .041 1(60 T430 .072 0.25 .33 16.64 .23 .034 2312 T-434 .052 0.74 .29 0.33 16.46 .80 .032 2313 T-30l .092 1.19 .28 7.32 17.72 .27 .033

To show the effect of carbon and nitrogen on the cold formability of the steel reference should be made to FIG. 1 of the drawings. As shown in FIG. 1, the designated alloys of TABLE 11 having varying carbon plus nitrogen contents were tested to determine roomtemperature tensile ductility as a measure of cold formability. As may be seen from this FIGURE at carbon plus nitrogen contents of less than 0.06 percent elongation values, and thus cold formability, are poor; whereas, with increased carbon plus nitrogen content, and particularly carbon plus nitrogen contents of above about 0.08 percent, the elongation values drastically increase. Although cold formability continues to increase with carbon plus nitrogen contents above about 0.14 and 0.13 percent, at these high levels of carbon plus nitrogen the hot workability of the alloy is adversely affected. This is demonstrated by the photographs shown in FIG. 2. As may be seen with Heats 1E57, 1E59 and 1E61 of TABLE II the hot workability of the samples shown in the photograph of FIG. 2 was excellent; whereas, with Heats 1E62 and 716, which had carbon plus nitrogen contents of 0.148 and 0.159, respectively, samples were not satisfactory from the standpoint of hot workability. Consequently, it may be seen from this data that although carbon plus nitrogen contents in increasing amounts are beneficial from the standpoint of achieving cold formability, nevertheless they must be restricted to about 0.13 or 0.14 percent to permit satisfactory hot working of the stainless steel.

The presence of manganese within the range of 8 to 16 percent, and preferably 9 to 13 percent, is likewise necessary to achieve the desired cold formability. This effect of manganese is shown in FIG. 3. Specifically as may be seen from this FIGURE to achieve an elongation in 2 in. of 30 percent a manganese content of at least 8 percent is required as demonstrated by Heat 2 of TABLE II. In comparison, as may be seen from FIG. 3, with Heat 1 of TABLE 11 having a manganese content of 5.87 percent and an otherwise substantially identical composition to Heat No. 2 the elongation in 2 in. was approximately 18 percent as compared with approximately 30 percent for Heat No. 2. As may be seen from the elongation values presented in FIG. 3 for Heats 4, 6 and 8, all of which have manganese contents approaching about 12 percent, each of these alloys was characterized by excellent cold formability as measured by the percent elongation in 2 in.

The elements silicon and nickel are present only in normal residual amounts. However, silicon contents of up to about 3 percent may be desirable for elevated temperature applications requiring a high level of oxidation resistance.

Molybdenum is critical from the standpoint of providing suitable corrosion resistance for motor vehicle applications. Specifically, for this purpose at least about 0.6 percent molybdenum, and preferably about 1.0 percent molybdenum, is needed in the steel of the invention to produce corrosion resistance comparable to that of A151 Type 434, which is a standard stainless steel grade having minimum acceptable resistance to the corrosive effects of road deicing salts that are the primary corrosive environment for stainless steels used in motor vehicle applications. As may be seen from FIG. 4 potentiodynamic polarization tests in chloridecontaining electrolyte indicate that about 0.90 percent molybdenum is required to produce pitting resistance comparable to Type 434. In the dip-dry tests, the results of which are shown in FIG. 5, it may be seen that with a residual molybdenum content of about 0.25 percent substantial pitting results; whereas, Type 434 is substantially free of pitting. It may be seen that with the samples constituting Heats 2310, 2311 having molybdenum contents of 0.71 and 0.90 percent, respectively, the pitting resistance improves over the lowmolybdenum containing samples constituting Heats 2314 and 2309. Particularly with Heat 2311 having a molybdenum content of 0.90 percent the sample shown in FIG. 5 of the drawing was completely free of pitting and equal to the Type 434 sample from this standpoint. As further evidence of the effect of molybdenum on corrosion resistance reference should be made to FIG. 6. As may be seen from these photographs of test samples that were in actual exposure on a passenger car driven in the Pittsburgh, Pennsylvania, area over the winter season 1969-1970 the corrosion resistance of the sample constituting Heat 2314 having residual molybdenum of about 0.25 percent was slightly poorer than the Type 434 sample; whereas, samples constituting Heats 2310 and 2311 having molybdenum contents of 0.71 and 0.90 percent, respectively, exhibited better corrosion resistance than the Type 434 sample. As would be expected from the above-reported data the sample constituting Heat 2311 having a molybdenum content of'0.90 percent showed essentially no corrosion and consequently was considerably superior to the Type 434 sample in this respect.

To obtain the optimum combination of hot workability and cold formability, it is necessary to balance the composition of the alloy with respect to the ferrite formers and austenite formers so as to produce substantially equal amounts of ferrite and austenite in the structure of the alloy when annealed typically within the range of 1,850 to 2,050F. In this regard reference should be made to FIG. 7 of the drawings wherein it is. shown that if the amount of the ferrite formers chromium, silicon, and molybdenum and the austenite promoters nickel, carbon and nitrogen are not properly balanced the corresponding balance of ferrite and austenite is not provided and consequently either hot workability or cold formability is impaired. More specifically in this regard, if the composition balance with respect to the ferrite formers and austenite formers is such that less than 30 percent ferrite is present, e.g., chromium equivalent of less than 13.5 percent, hot workability is poor and inadequate for satisfactory commercial production. If the composition balance is such that more than 65 percent ferrite is formed, e.g., chromium equivalent greater than 16.0 percent, the cold formability is decreased to a point where it is no longer superior to that of Type 434. From FIG. 7 it may be seen, therefore, that the various austenite and ferrite promoting elements in the composition must he carefully balanced to achieve the desired combination of hot workability and cold formability. In this regard, however, manganese is an exception in that the manganese content between 8 and 16v percent has no significant effect on the structure of the stainless steel from the standpoint of the austenite and ferrite content. Manganese is important, however, as described above from the standpoint of its effect on room temperature ductility. Also, in maintaining a balance between the austenite and ferrite producing elements to achieve a chromium equivalent of 13.5 to 16 percent it is necessary that chromium be present in an amount of at least percent from the standpoint of corrosion resistance. Amounts of chromium in excess of percent, however, are not necessary for this purpose and merely unduly add to the cost of the alloy.

I claim:

1. A duplex stainless steel having good hot workability, cold formability and corrosion resistance consisting essentially of, in weight percent, carbon 0.12 max., nitrogen 0.12 max., manganese 8 to 16, silicon 3 max., nickel 1.0 max., chromium 15 to 20, molybdenum 0.6 to 2.0, carbon plus nitrogen 0.06 to 0.14 and balance iron, with a chromium equivalent of 13.5 to 16.0 in accordance with the formulae:

Chromium Equivalent, in percent chromium silicon molybdenum nickel carbon nitrogen),

2. The steel of claim 1 having up to 10 percent silicon.

3. A duplex stainless steel having good hot workability, cold formability and corrosion resistance consisting essentially of, in weight percent, carbon 0.10 max., nitrogen 0.06 max., manganese 9 to 13, silicon 0.5 max., nickel 0.5 max., chromium 16 to 19, molybdenum 0.6 to 2.0, carbon plus nitrogen 0.08 to 0.13 and balance iron, with a chromium equivalent of 14.0 to 16.0 in accordance with the formulae:

Chromium Equivalent, in percent chromium silicon molybdenum nickel 30 carbon nitrogen),

to provide said steel with substantially equal amounts of ferrite and austenite after annealing within the range of 1,850 to 2,050F, whereby a combination of both good hot workability and cold formability is achieved.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,861,908 Dated 975 Inventor(s) Jerome Bressanelli It is certified that error appears in the aboveidentified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 13, change "of" to --or--;

Column 6, claim 2 line 1, change "10" to --l.O--.

Signed and sealed this 17th day of June 19-75.

(SEAL Attest C. I-IARSHALL DANN RUTH C. F-IASON Commissioner of Patents Attesting Officer and Trademarks FORM Po-1050 (10-69) USCOMM-DC 60376-P69 1' Hi5. GOVERNMENT PRINTING OFFICE: I969 0-366-334 

2. The steel of claim 1 having up to 10 percent silicon.
 3. A duplex stainless steel having gooD hot workability, cold formability and corrosion resistance consisting essentially of, in weight percent, carbon 0.10 max., nitrogen 0.06 max., manganese 9 to 13, silicon 0.5 max., nickel 0.5 max., chromium 16 to 19, molybdenum 0.6 to 2.0, carbon plus nitrogen 0.08 to 0.13 and balance iron, with a chromium equivalent of 14.0 to 16.0 in accordance with the formulae: Chromium Equivalent, in percent % chromium + % silicon + % molybdenum - % nickel - 30 (% carbon + % nitrogen), to provide said steel with substantially equal amounts of ferrite and austenite after annealing within the range of 1,850* to 2,050*F, whereby a combination of both good hot workability and cold formability is achieved. 